Rowing machines are used for exercise. They are made to simulate the motion one uses to row a boat. Rowing machines consist of a frame which is supported on the floor and upon which a seat that rides on a rail between a forward position and a backward position is mounted. Rowing machines have two simulated oar handles which can be pushed forward easily but must be pulled back against a resisting force. Although rowing machines provide excellent exercise for many muscles including legs, arms, back, shoulders and stomach they are not used consistently because the exercise is so boring.
Paraplegics and other persons with disabled legs use wheelchairs to move about. Wheelchairs are normally propelled by a propelling rim attached to the spokes of the wheels. The rims are positioned to be moved by grasping them and thrusting the arms forward. The propelling rims normally are smaller than the diameter of the wheels so that some mechanical advantage is obtained but it is not variable. Thus, whether propelling a wheelchair uphill, downhill or on a flat surface, a given motion of the user's arms and hands will produce the same amount of forward motion of the wheelchair. Wheelchairs are generally very slow moving and even those wheelchairs known to the art that have some variable mechanical advantage generally have only two or three speeds. Shifting between different speeds is usually accomplished in a manner analogous to multiple speed bicycles, specifically, shifting is accomplished by moving a drive chain between different sprockets.